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Composite video through modulator - Problem

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  • 28-04-2010 1:34pm
    #1
    Dr Pepper
    Registered Users Posts: 841 ✭✭✭


    Hi,

    Sorry in advance for the complicated, long story!

    I have a problem with composite video. I know it's old, poor-quality and probably fairly obsolete at this stage, however it is required for a project I'm working on at present.

    If I connect a composite cable directly from the PC to the TV's composite video-in port, it works fine and I can see the picture on the TV (i.e. the TV is detected by the graphics card and it produces a picture). If I then unplug that cable and feed the composite signal into an RF modulator, I get an RF picture out of the modulator and I can tune it in on my TV and it looks fine...

    However... If I connect a composite cable from the PC into the RF modulator initially and boot the PC, the graphics card doesn't detect the modulator and doesn't produce a picture from the composite port. This is what I'd like to do - Get the graphics card to always produce a composite picture whether it detects a TV or not.

    One solution that works is to solder a 75ohm resistor between the 2 composite contacts inside the PC. That way, it always detects a 75ohm resistance (and thinks there's a TV plugged in). The problem with that is: once you do connect a TV or modulator, the composite signal is badly affected by the extra (no-longer-required) resistance and the picture quality suffers (more).

    The resistance on the composite cable is 78ohms when attached to the TV and 1.1k ohms when attached to the modulator. I don't know much about electronics but I'm assuming this is why it's not detecting the modulator as a connected device (wrong resistance).

    Any advice? Told you it was a long story :P
    Tagged:


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    #2
    Dr Pepper
    Registered Users Posts: 841 ✭✭✭Dr Pepper


    Forgot to mention that it's an on-board, Intel Mobile 945 Express chipset.

    Anyone got any ideas?

    Here's an idea I have and perhaps people can help me develop/implement it...

    Add a resistor across the pins of the composite port but have also have a switch on the resistor that can be switched on at OS start-up so that the 'TV' can be detected, then switch it off again (so that it doesn't affect picture quality). Once the TV is detected once, the composite picture will continue to be produced while the PC is switched on. Of course, the switch should be controlled somehow by a script on the computer (maybe through the serial port?). Anybody got any advice on how best to do this?

    Cheers!


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    #3
    Solitaire
    Moderators, Technology & Internet Moderators Posts: 18,377 Mod ✭✭✭✭Solitaire


    You'd need to create a device that imitates the TV's detection signal/impedance and sends it in lieu of the missing one from the RF box. At its simplest it would be a composite male-female dongle with a couple of things hooked up to it so you don't need to go messing with the cables or ports themselves.


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